<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    div{
      width: 200px;
      height: 200px;
      background: red;
      /* opacity: .5;   */
      filter: alpha(opacity=50);
    }
  </style>
</head>
<body>
  <div></div>
  <!-- 
    IE6,7,8不支持opacity属性
    解决方法：
    filter: alpha(opacity=50)
   -->
</body>
</html>